Conventionally, various devices have been conceived as a radiation imaging device for imaging an internal structure of an object by making radiation transmit through the object. A commonly-used radiation imaging device is configured to image a radiation projection image by irradiating radiation to an object to make the radiation transmit through the object. In such a projection image, shading appears depending on the ease of permeation of radiation, which represents the internal structure of the object.
With such a radiation imaging device, only objects having a property capable of absorbing radiation to some extent can be imaged. For example, soft biological tissues hardly absorb radiation. Even if it is tried to image such a tissue with a general device, nothing is reflected in the projection image. When trying to image the internal structure of an object that does not absorb radiation as described above, there is a theoretical limit in a general radiation imaging device.
Under the circumstances, a radiation phase-contrast imaging device that images an internal structure of an object utilizing a phase-contrast of transmitted radiation has been proposed. Such a device is configured to image an internal structure of an object using Talbot interference.
Talbot interference will be described. From the radiation source 53 shown in FIG. 26, phase-aligned radiation is irradiated. When making the radiation transmit through the phase grating 55 which is in a streak form, the image of the phase grating 55 appears on the projection surface which is apart from the phase grating 55 by a predetermined distance (Talbot distance). This image is called self-image. The self-image is not just a projection image of the phase grating 55. The self-image occurs only at the position where the projection surface is separated from the phase grating 55 by the Talbot distance. The self-image is configured by the interference fringes caused by interference of light. The reason that the self-image of the phase grating 55 appears at the Talbot distance is that the phase of radiation generated from the radiation source 53 is aligned. When the phase of radiation is disturbed, the self-image appearing at the Talbot distance is also disturbed.
The radiation phase-contrast imaging device is configured to image an internal structure of an object utilizing the self-image disturbance. It is assumed that an object is placed between the radiation source and the phase grating 55. Since this object hardly absorbs radiation, most of the radiation incident on the object exits to the phase grating 55 side.
The radiation has not passed through the object completely as it is. The phase of the radiation changes while passing through the object. The radiation exited the object passes through the phase grating 55 with the phase changed. The observation of the radiation on the projection plane arranged at the Talbot distance shows disturbances in the self-image of the phase grating 55. The degree of disturbances of the self-image represents the radiation phase change.
The specific magnitude of the phase change of the radiation that transmitted through the object changes depends on where the radiation has transmitted through the object. If the object has a homogeneous configuration, the change of the radiation phase remains the same no matter where the radiation transmits through the object. In general, however, an object has some internal structure. When making radiation transmit through such an object, the phase change does not remain the same.
Therefore, when the phase change is known, the internal structure of the object can be grasped. The phase change can be known by observing the self-image of the phase grating 55 at the Talbot distance.
In such an apparatus, how to observe the self-image becomes a problem. The self-image has the same streak pattern as the pattern of the phase grating 55. The streak pattern needs to be considerably finer to the extent that Talbot interference occurs. It is technically extremely difficult to image such a very fine pattern. This is because a detector equipped with extremely small detection elements is required for the self-image detection.
Therefore, in some conventional configurations, there are configurations that give up detecting the self-image itself with detectors. That is, in a conventional configuration, as shown in FIG. 27, another grating (absorption grating 57) is set on a detection surface of a detector. The absorption grating 57 has a streak structure similar to the phase grating 55. Therefore, the self-image incident on the absorption grating 57 interferes with the absorption grating 57 to generate a moire. This moire has a pattern in which dark lines are arranged, and since the pitch between the dark lines is large, imaging can be sufficiently performed even if the size of the detection element is large. By detecting this moire, the self-image can be obtained indirectly (see, for example, Patent Document 1).